Essentially mangaka are just inefficient.
This was especially highlighted to me when reading some Chinese comics, where a fast one releases two chapters nearly every day. In color.
You could step in and remove the inefficiencies, but the root cause is that a lot of mangaka reach a point where their story is going nowhere and they reach that point relatively fast.
When that happens, either the mangaka has enough clout to go on hiatus or he hasn't and someone steps in to tell him how to continue the story. Usually by turning it into something formulaic, copying the formula of another ongoing popular work.
A solution would be for manga to copy anime and favor adaptations instead of original works, since if the story is already there, all the inefficiencies could be fixed.
But I guess the way things are everything is still profitable, so no one sees the need to fix the supposed problem.
I'm guessing that the manga industry will be steamrolled by the manhua industry (the Chinese comics) in the next two decades, simply because they release new things faster. So while you wait for the next chapter of your favorite manga, you can read a couple of thousand chapters of a bunch of manhua to pass the time waiting.